In recent years, in a long-distance optical communication in particular, an optical transmission system based on a digital coherent method has been developed by which the communication capacity per one channel can be increased dramatically. Such a system has been increasingly put to commercial use. In the field of optical communication based on a digital coherent method, a polarization multiplexing method has been generally used to give separate signals to two orthogonal polarizations to double the transmission quantity.
Various signal formats have been used to give a signal to each polarization. Among such formats, the one currently most actively put to commercial use includes systems having a communication capacity of 100 gigabit/second per a channel that are mostly based on QPSK (Quadrature Phase Shift Keying).
FIG. 1 illustrates the entire configuration of an optical transmission circuit and an optical reception circuit based on a digital coherent polarization multiplexing QPSK method according to the prior art. FIG. 1 illustrates a light source 9101 generating continuous light, the first optical power splitter 9102, an optical modulator 9103, and an optical demodulator 9104. In FIG. 1 and the subsequent drawings, the arrow of the solid line shows continuous light, the arrow of the dotted line shows modulated signal light, and the arrow of the double line shows an input and an output of an electric signal.
An optical transmission circuit and an optical reception circuit based on the coherent method are characterized in that the reception side also has a light source. A reference light inputted from the light source at this reception side and the reception signal light inputted from the transmission path are allowed to interfere each other, thereby detecting a reception signal at a higher sensitivity. Another configuration is also possible in which separate light sources are provided at both of the transmission circuit side and the reception circuit side, respectively. However, in recent years, as shown in the conventional example of FIG. 1, such a configuration has been mainly used in which a unified light source is used by branching the light by an optical power splitter to the transmission circuit side and the reception circuit side. This configuration is advantageous in size and power consumption.
The optical modulator 9103 of FIG. 1 receives a transmission electric signal and a continuous light from the light source 9101 branched by the first optical power splitter 9102. The optical modulator 9103 functions as a polarization multiplexing optical transmission circuit to modulate the continuous light based on the transmission electric signal to send a polarization multiplexed signal light to a transmission path.
The optical demodulator 9104 of FIG. 1 receives a polarization multiplexed signal light from the transmission path and a continuous light from the light source 9101 branched by the first optical power splitter 9102. The optical demodulator 9104 functions as the polarization multiplexing optical reception circuit that performs a coherent optical demodulation processing to output a reception electric signal.
FIG. 2 shows the details of the polarization multiplexing optical transmission circuit configured by the optical modulator 9103 of FIG. 1. The optical modulator 9103 has a system of two optical modulation circuits corresponding to two orthogonal polarizations. For convenience, the system of the two optical modulation circuits will be referred to as X polarization and Y polarization separate from actual polarization directions, respectively. FIG. 2 illustrates the second optical power splitter 9105, a Y polarization optical modulation circuit 9106, an X polarization optical modulation circuit 9107, a polarization rotator 9108, and a polarization beam combiner 9109.
The optical modulator 9103 receives continuous light having TE polarization for example from the first optical power splitter 9102. The inputted continuous light having TE polarization is branched to two continuous lights having TE polarization by the second optical power splitter 9105. The two continuous lights are modulated by the transmission electric signal in the Y polarization optical modulation circuit 9106 and the X polarization optical modulation circuit 9107, respectively.
The modulated TE polarization output from the Y polarization optical modulation circuit 9106 is converted to TM polarization output by the polarization rotator 9108. The converted TM polarization output and the modulated TE polarization output from the X polarization optical modulation circuit 9107 are multiplexed to a polarization multiplexed signal by the polarization beam combiner 9109. The polarization multiplexed signal is outputted to the transmission path.
FIG. 3 illustrates the details of a polarization multiplexing optical reception circuit configured by the optical demodulator 9104 of FIG. 1. As in the transmission circuit side, the reception circuit side also has two optical demodulation circuits corresponding to a system of two polarizations. FIG. 3 illustrates a polarization beam splitter 9111, a polarization rotator 9112, the third optical power splitter 9113, an optical coherent mixer 9114 as the first optical demodulation circuit, an optical coherent mixer 9115 as the second optical demodulation circuit, and photo detectors 9116 and 9117.
The optical demodulator 9104 receives a polarization-multiplexed reception signal light from the transmission path. The reception signal light is separated to a TE polarization component and a TM polarization component by the polarization beam splitter 9111. The continuous light provided from the light source 9101 is branched by the first optical power splitter 9102 as a reference light of TE polarization. The reference light is further branched to two components by the third optical power splitter 9113. These two components are inputted to two optical demodulation circuits configured by the optical coherent mixers 9114 and 9115.
The TE polarization component of the reception signal light separated by the polarization beam splitter 9111 and one of the reference lights of TE polarization branched by the third optical power splitter 9113 are inputted to the optical coherent mixer 9114 and are demodulated.
The TM polarization component of the reception signal light separated by the polarization beam splitter 9111 is converted to TE polarization by the polarization rotator 9112. The reception signal light component converted to TE polarization and the other of the reference lights of TE polarization branched by the third optical power splitter 9113 are inputted to the optical coherent mixer 9115 and are demodulated.
The demodulated signal light is converted to a reception electric signal by the photo detectors 9116 and 9117. The reception electric signal is outputted from the polarization multiplexing optical reception circuit.
Under the limited power light source condition, branching ratio of the continuous light from the light source 9101 between the transmission circuit side and the reception circuit side by the first optical power splitter 9102 generally provides superior transmission and reception characteristics when distributing higher power to the transmission circuit side than to the reception circuit side instead of using equal branching. The optimization of the branching ratio is detailed in the following Non-Patent Literature 1 (see FIG. 4 and Section 3) in which an asymmetry property between the transmission side and the reception side up to about 70:30 is appropriate depending on conditions (or the transmission distance of a signal light can be maximized).
The optical transmission circuit and the optical reception circuit based on the digital coherent polarization multiplexing method as described above are required to provide a further-smaller circuit in the future. Thus, research and development have been carried out to unify and integrate transmission and reception circuits.
For this purpose, an approach has been examined to use a Photonic Integrated Circuit (PIC) using an InP (indium phosphide) light waveguide or silicon light waveguide to integrate all optical circuit elements into one chip.
Specifically, an approach has been examined to provide a PIC (Photonic Integrated Circuit) by integrating all of optical modulation circuits 9106, 9107 and the optical demodulation circuits 9114, 9115 as well as optical power splitters 9102, 9105, and 9113, polarization beam combiner/splitters 9109, 9111, and the polarization rotators 9108, 9112 into one chip. Furthermore, another approach has been examined to integrate the light source 9101 and the photo detectors 9116, 9117. This configuration obtained by one chip integration also requires an asymmetric branching ratio of about 70:30 for the first optical power splitter 9102 to branch the continuous light from the light source 9101 between the transmission and reception circuits as described above.
Another currently-inevitable disadvantage is that, when the polarization rotators 9108 and 9112 are realized by PIC for one chip integration, the polarization rotator has an excessive circuit loss of about 1 to 2 dB. This excessive circuit loss in the polarization rotator causes the optical modulator 9103 constituting the polarization multiplexing optical transmission circuit of FIG. 2 to have a higher loss in the Y polarization optical modulation circuit 9106-side path passing through the polarization rotator 9108 than in the X polarization optical modulation circuit 9107-side path.
In order to correct this unbalanced loss between paths to minimize the loss of the entire optical modulator, the second optical power splitter 9105 also requires an asymmetric branching ratio at which a higher power is branched to the Y polarization optical modulation circuit 9106 side for example.
A means for realizing an optical power splitter in the PIC (Photonic Integrated Circuit) generally includes a multimode interference circuit or a directional coupler. However, the multimode interference circuit designed to have an asymmetric branching ratio involves a disadvantage that an excessive loss is increased when compared with a design having a 50:50 symmetric branching ratio.
The multimode interference circuit having the asymmetric branching ratio is previously examined for a quartz material waveguide. According to this examination result, when compared with the 50:50 symmetric design, the higher the asymmetry property is, the more accurately the power distribution ratios to the modes of the respective orders must be controlled.
However, a disadvantage of the excessive loss has been caused because a desired distribution ratio cannot be realized due to a manufacturing error. A material system used for PIC has a further-higher refractive index than that of quartz material, thus causing a smaller tolerance to the manufacturing error and further worsening the above-described disadvantage of the excessive loss.
This disadvantage of the excessive loss undesirably causes another excessive loss because while an asymmetric branching ratio is designed in order to reduce the loss of the entire circuit by compensating the unbalanced loss depending on the path, thus inevitably failing to reduce the loss of the entire circuit.
In the case of designing an asymmetric branching ratio by using a directional coupler, the principle of the directional coupler causes another disadvantage. It is that, the directional coupler of an asymmetric branching ratio has, when compared with the directional coupler of 50:50 symmetric branching ratio, dependency on the wavelength. In this case, although the directional coupler design having the asymmetric branching ratio can reduce the loss of the entire circuit at a specific wavelength, this design cannot compensate the unbalanced loss depending on the path at another wavelength of the operation wavelength range, thus inevitably failing to reduce the loss of the entire circuit.
As described above, the polarization multiplexing optical transmission and reception circuit intended for one chip integration has a disadvantage that the use of an optical power splitter having a symmetric branching ratio causes an unbalanced loss depending on the path. When an optical power splitter having an asymmetric branching ratio is used in order to compensate for the unbalanced loss depending on the path, a disadvantage is caused in that another excessive loss is caused by the characteristic of the optical power splitter itself.